In a large bowl, combine the water, mayonnaise and vanilla until well blended.
In a large bowl, combine the flour, sugar, cocoa, baking soda and salt; gradually beat into mayonnaise mixture until blended.
Pour into greased 9-in. square or 11-in. x 7-in. baking pan. Bake at 350° for 30-35 minutes or until a toothpick inserted near the center comes out clean. Cool completely